Special Issue: Vectors

ReviewInsecticide Resistancein African AnophelesMosquitoes: A WorseningSituation that Needs UrgentAction to Maintain MalariaControlHilary Ranson1,* and Natalie Lissenden1

Malaria control is reliant on insecticides to control the mosquito vector. Asefforts to control the disease have intensified, so has the selection pressure onmosquitoes to develop resistance to these insecticides. The distribution andstrength of this resistance has increased dramatically in recent years and nowthreatens the success of control programs. This review provides an update onthe current status of resistance to the major insecticide classes in Africanmalaria vectors, considers the evidence that this resistance is alreadycompromising malaria control efforts, and looks to the future to highlight someof the new insecticide-based tools under development and the challenges inensuring they are most effectively deployed to manage resistance.

A Limited Toolbox for Malaria Vector ControlIncreased coverage of those at risk from malaria with long-lasting insecticidal nets (LLINs)(see Glossary), and to a lesser extent indoor residual spraying (IRS) with insecticides,together with improvements in case management have had an enormous impact, halvingmalaria deaths and decreasing disease incidence by over a third since the beginning of thecentury [1,2]. These interventions are reliant on a small number of insecticides, with just thepyrethroids available for LLINs, and therefore the emergence and spread of insecticide resis-tance in African malaria vectors is a critical threat to malaria control. Although increases in levelsof pyrethroid resistance in Anopheles mosquitoes are widely documented, opinion is divided onthe current and future impact of this resistance on efforts to reduce or eliminate malariatransmission. This review outlines the challenges in defining the impact of resistance, assessesthe strength of the current evidence, and considers the future prospects for malaria control ifresistance eventually renders the pyrethroid insecticides obsolete.

A Rapidly Changing LandscapeThe distribution of pyrethroid resistance in African malaria vectors was described in a 2011article in this journal [3]. At this time, pyrethroid-resistant populations of Anopheles gambiae were

TrendsResistance to pyrethroid insecticides,the only class available to treat bednets, is now ubiquitous in Africanmalaria vectors and resistance to otherinsecticide classes used for adult mos-quito control is increasing.

Critical knowledge gaps impede esti-mates of the impact of this resistanceon malaria transmission but multipleobservational studies suggest a rapidlyworsening situation.

New approaches to tackle pyrethroidresistance are urgently needed; this willrequire new products, a more rapidand robust approach to their field eva-luation, and acceleration of access tothese products where the malaria con-trol challenges posed by resistance aregreatest.

1Department of Vector Biology,Liverpool School of Tropical Medicine,Pembroke Place, Liverpool L3 5QA,UK

*Correspondence:Hilary.Ranson@lstmed.ac.uk(H. Ranson).

Trends in Parasitology, March 2016, Vol. 32, No. 3 http://dx.doi.org/10.1016/j.pt.2015.11.010 187© 2015 Elsevier Ltd. All rights reserved.

prevalent in western and central Africa but were rarer in southern and eastern countries of thecontinent. Pyrethroid resistance is now widely spread across the continent with An. gambiae inKenya, much of Tanzania, Zambia, and Zimbabwe resistant to this insecticide class. Data remainscarce for much of central Africa, although reports are emerging of pyrethroid resistance acrossthe Democratic Republic of Congo [4]. In summary, although An. gambiae populations fullysusceptible to pyrethroids are still present in 2015 [e.g., in parts of Angola, Madagascar, andMozambique (http://www.africairs.net/wp-content/uploads/2014/11/Multi-Country-Profile-of-Insecticide-Resistance-on-Malaria-Vectors.pdf)] they are becoming increasingly outnumberedby resistant populations (Figure 1A).

Data on resistance in Anopheles funestus remain limited (Figure 1B) but, whereas previouslypyrethroid resistance in this species was thought to be restricted to southern Africa, it has nowbeen detected in Uganda [5], Kenya [5], Benin [6], and Cameroon [7]. As with An. gambiae, fullysusceptible populations of An. funestus have been reported in some areas of Mozambique(Figure 2D) but resistance is very prevalent in other areas of the country [8]. Furthermore, thetrend is similar to that observed in An. gambiae, with susceptible populations becoming theexception rather than the norm.

Several countries have established longitudinal monitoring in sentinel sites enabling temporalchanges in the prevalence of resistance to be detected. Examples from Malawi, Mozambique,Tanzania, and Uganda are shown in Figure 2. Changes in the methods and timing of mosquitocollections, which may in turn alter the relative proportion of morphologically identical siblingspecies used in the bioassays, will undoubtedly influence the results and make estimates about therate of spread of resistance difficult to infer from bioassay data alone. Furthermore, resistance canbe remarkably focal [9,10]. Nevertheless, three of the sites shown in Figure 2 exemplify the patternacross large parts of Africa in which 10 years ago control programs were targeting vectorpopulations that had full susceptibility to pyrethroids but are now facing the challenge of overhalf of the potential malaria vectors having developed resistance to this insecticide class.

The data in Figures 1 and 2, and indeed the vast majority of data currently being collected oninsecticide resistance in mosquitoes, indicate the response of the mosquito population to adiagnostic dose of insecticide, but this dose bears little relationship to the field dose of

GlossaryAnopheles gambiae sensu lato(An. gambiae s.l.): nomenclatureused to characterize species of theAnopheles gambiae complex, agroup of eight distinct siblingmosquito species that aremorphologically identical but exhibitdifferent behavioral traits.Exophilic: term used to describemosquitoes that generally reside/restoutdoors after taking a blood meal.Experimental hut: standardizedstructures that act as a proxy for localhouses. Used in the evaluation ofvector control tools that target indoor-biting mosquitoes. Modifications tothese structures, such as closedeaves and window exit traps, allowstandardized collection ofentomological end points such asmosquito mortality and deterrency.Indoor residual spraying (IRS): avector control intervention thattargets indoor-biting mosquitoes.Long-lasting insecticides are appliedto wall surfaces inside houses viaspraying and susceptible mosquitoesare killed when they come intocontact with these surfaces.Longitudinal monitoring: field studyof mosquitoes over time. Informationsuch as mosquito species,abundance, distribution, andsusceptibility to insecticide arestudied and collated.Long-lasting insecticidal nets(LLINs): mosquito nets withpyrethroid insecticides incorporatedinto their fibers. Susceptiblemosquitoes are killed on contact withthe net's surface. Effective forapproximately 3 years.Randomized controlled trial: astudy used to evaluate a specificintervention, drug, or strategy (e.g.,mosquito net, malaria prophylaxis).Individuals or groups are assigned atrandom to one of the interventionsunder study, with one study armreceiving a control (or placebo)intervention.Sentinel sites: key geographicallocations where extensive informationis collected and collated to helpinform control programs and policydecisions affecting largergeographical areas.Slide positivity rate: an alternativemeasure for malaria incidence.Defined as the number of laboratory-confirmed (using light microscopy)malaria cases per 100 suspectedcases examined.

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Figure 1. Changes in Pyrethroid Mortality in Major African Malaria Vectors Over Time. Percentage mortality of (A)Anopheles gambiae sensu lato (s.l.) and (B) Anopheles funestus s.l mosquitoes exposed to 0.05% deltamethrin (blue) or0.75% permethrin (orange) in World Health Organization (WHO) susceptibility bioassays. Data from 1995 to 2015 wereextracted from IR Mapper [21] in August 2015 and supplemented with a literature search for 2014 and 2015 data. Each dotrepresents a data point extracted from IR Mapper or from the literature search and the dotted lines show trend lines for themortality rates for each insecticide.

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insecticide. Alternative measures of resistance in which the intensity or strength of the resistanceis measured have been described [11] and are being adopted in a small number of field studies[12–14]. Again, longitudinal monitoring of resistance intensity can provide an important insighton the rapid changes occurring in malaria vectors. For example, the exposure time required tokill 50% of the An. gambiae population in an area in southwestern Burkina Faso was foundto increase tenfold over a single year [13].

Perhaps of more direct relevance in decision making are assays that measure the response oflocal vectors to locally implemented vector control tools. The simplest example of this is the conebioassay in which mosquitoes are exposed to a bed net or a sprayed wall for a fixed exposuretime and then mortality recorded 1 h and 24 h after exposure [15]. Results from cone bioassayspaint an alarming picture, with very low kill rates being observed even after exposure to new netsor freshly sprayed surfaces in several settings [8,11,13,16–18].

Increasing Reports of Resistance to Other Insecticide ClassesMost An. gambiae sensu lato (s.l.) populations remain susceptible to carbamates (Figure 3A)and organophosphates (Figure 3B), although reports of resistance to these two classes [whichshare the same mode of action (MoA)] are increasing and may be expected to rise further in areaswhere IRS programs are replacing pyrethroids with these insecticide classes in response topyrethroid resistance (see below). Of particular concern are populations that show resistanceto all four classes of insecticide available for malaria control, this has been reported in severalcountries including Côte d’Ivoire [19] and Mali [10].

Keeping track of the spread of insecticide resistance is a challenge for malaria control programs,with many lacking the resources or expertise to conduct regular monitoring and/or the

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Figure 2. Changes in Mosquito Mortality Over Time. Percentage mortality of Anopheles gambiae sensu lato (s.l) from(A) Tororo, Uganda and (B) Arumeru, Tanzania and of Anopheles funestus s.l from (C) Chikwawa, Malawi and (D) Mocuba,Mozambique. Mosquitoes were exposed to 0.05% deltamethrin (blue) or 0.75% permethrin (orange) in World HealthOrganization (WHO) susceptibility bioassays. Data labels show number of mosquitoes exposed per assay. Data wereextracted from the IR Mapper [21] August 2015 database. NR, not recorded. When multiple data points existed for a givenlocality for a particular year the bioassay with the largest sample size was selected or a random number generator was usedto select the result.

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databases to store and access this data [20]. Two global databases have been established (IRMapper [21] and VectorBase [22]), with the WHO also planning to establish their own globaldatabase [20]. These databases face difficulties in ensuring the timely and comprehensiveinclusion of quality-assured data from all sources but an even greater obstacle for countries isusing the available resistance data to make informed decisions on malaria control. With just onechemical class available for LLINs and two MoAs for IRS, limited budgets, and widespreadresistance already circulating, preserving the susceptibility of malaria vectors is probably beyondthe reach of most control programs. As a result, decisions on changes in insecticide use aregenerally precluded by evidence of control failure with existing tools. A greater understanding ofthe link between resistance bioassay results and the protective efficacy of LLINs or IRS isimportant to facilitate this decision making and also to predict the future prospects for malariacontrol in an era of widespread resistance.

What Is the Impact of Insecticide Resistance on Malaria Transmission?Measuring the public health impact of insecticide resistance is critical for assessing the changingdynamics of malaria transmission across Africa and mobilizing resources to tackle resistance.There are major challenges in quantifying this relationship (Box 1) and even if it were possible todesign and implement appropriate randomized controlled trials to assess the impact ofresistance, these would be unlikely to yield answers in the time frame necessary for action. Belowwe review some of the alternative approaches to assessing the impact of pyrethroid resistance.

Experimental hut data could provide information on the impact of resistance on both thepersonal protection (via blood-feeding inhibition) and the community protection (by increasedmosquito mortality) afforded by insecticides, if comparable studies were conducted in areas

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Figure 3. The Distribution of Carbamate (A) and Organophosphate (B) Resistance in Africa. Maps were createdfrom data extracted from the IR Mapper [21] August 2015 database, using data collected between 2011 and 2015 fromWorld Health Organization (WHO) susceptibility and Centers for Disease Control and Prevention (CDC) bottle bioassays.Data were omitted if the insecticide dose tested was not recommended by the WHO or CDC [44] or if the concentration wasnot recorded.

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differing in their resistance phenotype. A recent systematic review of experimental hut studiesevaluating LLIN performance demonstrated a small but significant impact of pyrethroid resistanceon these entomological indicators [23], but most studies included in this review were conductedbefore the most potent pyrethroid-resistance mechanisms were widely established and hencemay underestimate the current situation across Africa. Mathematical models of malaria transmis-sion can be used to translate entomological outcomes from experimental hut trials into estimatesof the number of additional malaria cases due to resistance. A study employing this approachfound a positive correlation between insecticide susceptibility status and protection against bloodfeeding by LLINs [24] and concluded that LLINs would avert up to 40% fewer episodes of malaria inthe most resistant areas compared with areas with a fully susceptible population. More recently,modeling of the outcomes of a study in which susceptible or resistant mosquitoes were releasedinto experimental huts containing holed LLINs concluded that the impact of LLINs on reducingmalaria transmission was dependent on the level of resistance in the population [25]. A similarconclusion resulted from an earlier study measuring entomological indicators in householdsusing LLINs in areas of Benin where the vectors were either susceptible or resistant to pyrethroids.Sleeping under a LLIN was no more protective than sleeping under an untreated net, regardlessof its physical condition, in areas with high pyrethroid resistance [26].

A further indicator that resistance may be compromising the efficacy of control tools is providedby studies reporting the collection of sporozoite-infected mosquitoes either resting on wallsnewly treated with IRS or inside LLINs [27]. An extension of this approach is to test forassociation between insecticide resistance markers and Plasmodium infection in wild-caughtmosquitoes. If resistance is enabling mosquitoes to survive repeated insecticide exposure, theprevalence of sporozoites would be expected to be higher in mosquitoes containing insecticide-resistant alleles; the development of further molecular markers of resistance will facilitate studiesof this nature [28].

Under operational settings the impact of insecticide resistance on malaria transmission will beinfluenced by a large number of factors including those unrelated to the vector itself (e.g., the

Box 1. Challenges in Assessing the Impact of Resistance on Malaria Transmission

Extrapolating from bioassay data indicating that insecticides are killing a smaller proportion of mosquitoes to estimates ofthe impact of this resistance on malaria control tools is not straightforward. Some of the potential challenges andconfounding factors include the following.� Resistance is a variable trait. The phenotype is influenced by, for example, the age and physiological status of themosquito [45], the rearing conditions of the larvae before the assay, and the temperature and humidity in the testingroom [29]. This introduces large variability into bioassay data, making significant trends difficult to detect.� Bioassays do not capture the lifetime impact of insecticide exposure. Routine surveillance typically assessesresistance on the basis of 24-h mortality responses. However, if resistant mosquitoes have reduced fitness aftersurviving exposure to LLINs, the impact of resistance on parasite transmission may be reduced.� LLINs provide a physical and chemical barrier to mosquitoes. Intact LLINs may still provide a high degree of personalprotection even when most mosquitoes are resistant. Yet, once holes appear or insecticide efficacy declines with netage, resistant mosquitoes will have a greater competitive advantage. Studies should evaluate the performance of LLINsthat have been in use for a minimum of 1 year under field conditions.� The interactions between mosquito behavior and insecticide resistance are very poorly understood. Does resistanceimpact the mosquitoes’ ability to detect a blood meal? Are resistant mosquitoes less likely to avoid an insecticide-treatedsurface? Resistance should be evaluated for its impact on all of the key behavioral traits that influence vectorial capacity[46,47].� The role of multiple vectors is overlooked. Most studies look at resistance in the species that is easiest to collect(typically Anopheles gambiae s.l.) and ignore the role of other major or minor vectors.� The perfect study to assess the epidemiological impact of resistance is probably impossible to implement. Forexample: resistance is a constantly evolving trait and cannot be randomized; for ethical reasons it is not practical towithhold an intervention from one study arm to assess the level of protection added by insecticides; and the lack oflongitudinal data on resistance generally precludes robust assessments of the changing impact of interventions asresistance emerges.

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efficacy of case management approaches, drug resistance). Ongoing observational studies areattempting to assess the impact of resistance by comparing malaria transmission across areaswhere vector populations differ in susceptibility to insecticides [29]. However, the challenges ofthis approach are manifold, particularly due to the confounding effect of differing transmissionintensity and vector ecology across sites.

Longitudinal studies, with accurate records of malaria transmission and resistance levels, mayprovide the best opportunity to observe the impact of resistance. The most widely cited evidencefor the impact of resistance comes from such a study in KwaZulu Natal, which demonstrated acorrelation between the emergence of pyrethroid resistance and a spike in malaria cases thatwas later contained by the reintroduction of dichlorodiphenyltrichloroethane (DDT) [30]. Morerecently a similar conclusion was reached in Senegal, where reduction in LLIN efficacy wasattributed to resistance, although the absence of longitudinal resistance data makes thisconclusion difficult to validate [31]. Unfortunately, the opportunity for initiating new studies ofthis nature, at least for pyrethroid resistance in Africa, may have passed, unless good historicaldata sets already exist.

Indirect evidence that insecticide resistance is impacting malaria transmission can be obtainedfrom retrospective analysis in countries that have changed insecticide class in IRS programs(usually in response to either reports of resistance or increases in malaria cases) and seen animprovement in control. As an example, DDT and pyrethroids were being used for IRS in Ugandadespite the known presence of resistance [32]. When these insecticides were replaced with thecarbamate bendiocarb, a marked improvement in slide positivity rates was observed [32].Similarly, in Ghana, pyrethroid resistance triggered a switch to the use of the organophosphateinsecticide Actellic (primiphos-methyl) for IRS that was associated with a noticeable impact onkey indicators of malaria transmission such as the number of children with parasitologically orclinically diagnosed malaria (Figure 4) (http://www.pmi.gov/docs/default-source/default-document-library/malaria-operational-plans/fy-15/fy-2015-ghana-malaria-operational-plan.pdf?sfvrsn=3).

Despite the rapid emergence of resistance, it is important to remember that sleeping under anLLIN still provides protection from malaria, even in areas with pyrethroid resistance [33].

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Figure 4. Improvement in Malaria Indi-cators in Bunkpurugu-Yunyoo,Ghana. Data were collected between2010 and 2013 after a switch in insecticideclass from pyrethroids to organopho-sphates for indoor residual spraying [Pre-sident's Malaria Initiative (2015) GhanaMalaria Operational Plan FY 2015 (http://www.pmi.gov/docs/default-source/default-document-library/malaria-operational-plans/fy-15/fy-2015-ghana-malaria-operational-plan.pdf?sfvrsn=3)].Arrow indicates when spraying with theorganophosphate Actellic CS was intro-duced. RTD, rapid diagnostic test.

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However, vital questions remain to be answered about how much of the protection is due to thephysical barrier of the net itself and how the longevity of the protective response (at both thepersonal and community level) is affected by resistance.

The Response of Control Programs to the Emergence of ResistanceIn 2012, the WHO published the Global Plan for Insecticide Resistance Management in malariavectors (GPIRM), which provides a series of recommendations on action to take if resistance isdetected [34]. The options for programs reliant on LLINs are clearly very limited given that thereare currently no alternative insecticides for net impregnation and the WHO policy, adopted by themajority of countries in Africa, is to achieve universal LLIN coverage (http://www.who.int/malaria/publications/atoz/who_recommendations_universal_coverage_llins.pdf?ua=1). Essentially, therecommendation is to continue as is if the resistance mechanisms do not impact control butintroduce IRS if more potent metabolic resistance mechanisms are present. However, theintroduction of IRS is beyond the financial capabilities of many control programs. Furthermore,the benefits of combining the two interventions are far from clear. A meta-analysis of datafrom 11 countries found significantly reduced parasitemia when the two interventions werecombined in areas of medium or high transmission [35], but this study did not includeinformation on the resistance status of the vectors in the various sites. More recently, severalrandomized controlled trials have been conducted and, interestingly, no benefit was observedby combining LLINs with carbamate or DDT IRS in two studies with resistant vectors [36,37].However, in a trial in Tanzania, where the more exophilic Anopheles arabiensis is animportant vector, there was significant additional benefit from combining the interventions[38].

For malaria control programs using IRS, it should theoretically be possible to manage insecticideresistance by careful preplanned rotation of insecticide classes with different MoAs (i.e.,alternating between DDT or pyrethroids and carbamates or organophosphates). The WHOrecommends that pyrethroids are not used for IRS, to reduce the selection pressure onmosquitoes to develop resistance to this class, and many programs have reduced their relianceon this insecticide class either in response to this guidance or due to the loss of control frompyrethroid-based IRS. As an example, the President's Malaria Initiative Africa Indoor ResidualSpraying (PMI AIRS) program was operational in 11 countries in 2015, with all programsspraying with carbamates or organophosphates and only one including limited spraying withpyrethroids. Contrast this with the situation in 2011, when 11 of 16 active programs utilizedpyrethroids. However, there are financial and logistical challenges associated with switching toan alternative insecticide class and the increase in cost of alternative chemistries is leading toreductions in the number of households protected by IRS [1,20]. Furthermore, some programsthat have switched from pyrethroids to carbamates for IRS have witnessed a rapid rise ofresistance to the latter class [39].

Future ProspectsNew insecticides are urgently needed to counteract the rapid emergence of resistance andsustain efforts to drive malaria transmission to zero. A product development partnership, theInnovative Vector Control Consortium (IVCC), is on track to deliver its target of three new publichealth pesticides but these are unlikely to reach the market before 2020 (Figure 5). Meanwhile,there is hope that additional classes of insecticides will become available for the control of adultmosquitoes via the reformulation of insecticides already in use in agriculture, mostly targeted forIRS. In 2015, two agrochemical companies announced their intention to market new IRS and/orLLIN products containing previously registered agricultural insecticides (http://www.publichealth.basf.com/agr/ms/public-health/en_GB/content/public-health/our-partners/malaria_control/beating_insecticide_resistance; http://sumivector.com/news/novel-mode-of-action-indoor-residual-spray-irs-product-from-sumitomo-chemical-shows-long?

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utm_source=emailcampaign224&utm_medium=phpList&utm_content=HTMLemail&utm_campaign=Novel+mode+of+action+indoor+residual+spray+%28IRS%29+product+from+Sumitomo+Chemical+shows+long+lasting+biological+activity).

In addition, net manufacturers have developed new LLINs that contain the pyrethroid synergistpiperonyl butoxide (PBO) and others are developing nets containing multiple active ingredients[40,41]. Experimental hut trials suggest that these LLINs provide better protection againstpyrethroid-resistant mosquitoes than conventional LLINs [41,42] but they have yet to beevaluated in large-scale field trials. Their use may help maintain the personal and communitybenefit of LLINs in areas with pyrethroid-resistant vectors until novel public health insecticidesbecome available.

It is critical that future insecticide-based approaches are not dependent on a single activeingredient in the way we have been reliant on the pyrethroids since the scale up of malaria controlefforts in 2000. There must not be a race to be first to the finish line in the introduction of newinsecticides into the marketplace. This would provide a very short-term solution and inevitablylead to the same issues now being faced with pyrethroid resistance. This is the primary driverbehind the IVCC's target to develop three novel public health insecticides, allowing for combi-nation and rotation strategies that will optimize performance and reduce the likelihood ofresistance developing. However, in addition to new insecticides a multitude of supportingactivities are needed to maximize the time until resistance undermines the efficacy of any ofthese new chemistries. Manufacturers, donors, control programs, the WHO, and other stake-holders must work together to develop and implement resistance management strategies.

New chemistry is only part of the picture. Alternative approaches that reduce our reliance onchemical insecticides are needed, not least to tackle transmission that occurs outside thehome where LLINs or IRS are not protective (http://www.who.int/malaria/publications/atoz/technical-note-control-of-residual-malaria-parasite-transmission-sep14.pdf).

Concluding RemarksPyrethroid resistance is ubiquitous in African malaria vectors and is rapidly increasing in strengthin many regions. Relatively little is known about the fitness costs of resistance althoughunselected mosquito colonies can maintain their resistance in insectaries suggesting that someof the highly resistant phenotypes now selected for in the field may be stable traits [43]. Even ifpyrethroid-resistant mosquitoes were less competitive than their susceptible counterparts when

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Figure 5. The Innovative Vector Control Consortium (IVCC) Pipeline of Novel Active Ingredients Currently inDevelopment. AI, active ingredient; PoC, proof of concept; VC, vector control; WHOPES, WHO Pesticide EvaluationScheme. A–C indicate the three lead compounds in predevelopment (pre-dev.). Image courtesy of the IVCC.

Outstanding QuestionsWhat is the most informative measureof insecticide resistance? Do currentdiscriminating dose bioassays providesufficient information or should they besupplemented by an additional test for‘operationally significant’ resistance?

Can molecular markers of resistanceinform decision making in insecticideselection and resistance management?

How does insecticide resistance influ-ence mosquito behavior and fitness?How does this affect the impact ofresistance on malaria transmission?

What is the resistance ‘breakpoint’above which LLINs provide only aphysical barrier against mosquitobites? Has this already been reachedin some settings?

What is the economic impact of insec-ticide resistance? Can we put a mone-tary value on preserving susceptibilityand, if so, who should bear this cost?

How should new public health insecti-cides be introduced to delay the emer-gence of resistance? Can modelinghelp predict the dynamics of insecticideresistance and develop practical strat-egies to minimize resistance selection?

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selection pressure was removed, it is unlikely that resistance alleles would be rapidly selectedagainst if malaria control programs ceased use of this insecticide class. This is becauseAnopheles mosquitoes are continually exposed to pyrethroids via their use in agriculture andhousehold products (e.g., aerosol sprays, coils). Besides, with hundreds of millions of LLINs inuse in Africa, and no non-pyrethroid LLIN products expected for 5 years or more, pyrethroidresistance can only be expected to increase.

Although there are many indicators that pyrethroid resistance is already compromising control,indisputable evidence is lacking. Yet given the complexities in measuring the impact of insecti-cide resistance (Box 1) we cannot equate lack of evidence of impact with evidence for no impact.LLINs in good physical condition undoubtedly still provide protection against malaria and thespread of resistance should not derail plans to increase access to the most effective tool toreduce malaria transmission. However, the malaria community cannot afford to be complacentabout insecticide resistance. Critical knowledge gaps on the causes and consequences ofinsecticide resistance need to be filled (see Outstanding Questions), the development, evalua-tion, and implementation of new products must be accelerated, and an evidence base for howbest to deploy insecticide to minimize the spread of resistance must be generated to ensure thesuccess of future malaria control efforts.

AcknowledgmentsThe authors thank Dr Nick Hamon, CEO of IVCC, for helpful comments on the manuscript and Dr Christen Fornadel, USAID,

for sharing information on insecticide use by the President's Malaria Initiative and permission to use the data in Figure 4. The

financial support of the European Union Seventh Framework Programme FP7 (2007–2013) under grant agreement no

265660 AvecNet is gratefully acknowledged. N.L. was supported by an ISSF Grant from the Wellcome Trust.

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